Inheritance

[Adam Chalkley]

Hi I'm having a problem for some reason I am getting a java.lang.NoSuchMethodError exception when I run my program but everything compiles fine,I understand inheritance but for some reason I'm getting this message here is my code.

class mm1{

	public static void main(String [] args){

		mm2 one = new mm2(23,"adam");
		mm3 two = new mm3();

		one.sayhello();
		two.sayhello();
	}
}

class mm2{

	private int age;
	private String name;

	public mm2(int x, String y){

		setage(x);
		setString(y);
	}

	public void sayhello(){

		System.out.print("hello ");
	}

	public void setage(int x){
		age = x;
	}

	public int getage(){
		return age;
	}

	public void setString(String y){

		name = y;
	}

	public String getString(){

		return name;
	}
}

class mm3 extends mm2{

	// nothing in this class just for testing

	public void color(){

	}

}

the exact error is java.lang.NoSuchMethodError: mm2: method(init)() not found

at mm3.(init)(mm3.java:1)
at mm1.main(mm1.java:7)


Thanks guys any help would be appreciated =) thanks

[Henry Wong]

Does the code compile? ... because you really shouldn't be running code that doesn't compile.

Henry

[Campbell Ritchie]

NoSuchMethodError(←link) suggests you compiled the code, then removed a method from one class and compiled that class on its own. Now you have removed a method and the rest of the code won't work.

[Adam Chalkley]

Campbell Ritchie wrote:NoSuchMethodError(←link) suggests you compiled the code, then removed a method from one class and compiled that class on its own. Now you have removed a method and the rest of the code won't work.

the code compiles just as it should, but I didn't remove any methods.I thought that once you declare a public method in one class and if you make it a super class of another by using the keyword extends in the subcless you can you that superclasses method which I called in the main program but get that exception.

[Henry Wong]

Adam Chalkley wrote:the code compiles just as it should,

The mm3 class does *not* compile, as the mm2 superclass doesn't have the required no-arg constructor.

Adam Chalkley wrote: ... but I didn't remove any methods.

I will speculate that you recently added a constructor to the mm2 class -- which of course, will remove the default no-arg constructor. And you forgot to go back to recompile your mm3 class.

Henry

[Adam Chalkley]

Henry Wong wrote:

Adam Chalkley wrote:
the code compiles just as it should, but I didn't remove any methods.

The mm3 class does *not* compile, as the mm2 superclass doesn't have the required no-arg constructor.

Henry

how come I need a constuctor for that to work? I'm not passing any arquements I'm just trying to print a method from the sub class which should have been inherited from the super class? plus I thought if a no arg constructor is not made that java makes a default no args constuctor

[Henry Wong]

Adam Chalkley wrote: ... plus I thought if a no arg constructor is not made that java makes a default no args constuctor

Incorrect. If no constructor (at all) is provided then a default no-args constructor is provided. If any constructor is provided, then the java compiler doesn't add any constructor.

BTW, why don't you just try compiling the mm3 class again? You will see that it doesn't compile.

Henry

[Adam Chalkley]

Henry Wong wrote:

Adam Chalkley wrote: ... plus I thought if a no arg constructor is not made that java makes a default no args constuctor

Incorrect. If no constructor (at all) is provided then a default no-args constructor is provided. If any constructor is provided, then the java compiler doesn't add any constructor.

BTW, why don't you just try compiling the mm3 class again? You will see that it doesn't compile.

Henry

yup did not compile at all,did after I added the default constructor,but how come I needed a default constructor on my class mm2 and not need a constructor on mm3 as I'm just calling a method from the superclass?

[Paul Clapham]

You're not just calling a superclass method in your mm3 class -- the method isn't static, so you can't call it until you construct an mm3 object. And for that you need a constructor. You don't have one, so the compiler inserts a default zero-argument constructor, but that still isn't good enough because that default constructor tries to call the zero-argument constructor of the superclass. Which it can't, because there isn't one.